There has been a problem with a conventional fluorescent lamp in that its long-time operation results in blackening of a glass tube and reduced luminous flux. It is considered that such blackening is caused, for example, by the factors as follows: (1) reaction between mercury enclosed in the fluorescent lamp and alkali components in the glass tube forms amalgam; (2) metal mercury penetrates into the glass tube; and (3) solarization occurs due to ultraviolet ray. Thus, it is known that blackening of a glass tube is prevented and reduction in luminous flux is inhibited by forming a protective film comprising metal oxides between the glass tube and a fluorescent layer.
It is desired that the protective film is a dense, homogeneous film, which does not have non-uniformity or falling-off, so that its function as a protective film can be fully displayed. Generally, fine particles with an average particle diameter of about 1 .mu.m or less are used as the metal oxides in such a protective film. The fine particles are uneven in shapes, and they have a needle-shape, or a dendritic or feather-like shape. Because such fine particles tend to aggregate easily, preparation of a suspension of these particles with good dispersibility has been difficult. When a suspension not fully dispersed is applied to a glass tube, it is likely that pinholes are developed due to non-uniform coating or falling-off of the particles. Particularly, a thin film tends to be significantly non-uniform and become discontinuous, so that it cannot fully display its function as a protective film. On the other hand, a film with increased thickness tends to generate cracks or falling-off, so that its function as a protective film cannot be obtained adequately.